Generally, during operation of a wind turbine, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft drives a gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed, wherein the high-speed shaft rotatably drives a generator rotor. In many conventional wind turbine configurations, the generator is electrically coupled to a bi-directional power converter that includes a rotor-side converter (RSC) joined to a line-side converter (LSC) via a regulated DC link. The LSC converts the DC power on the DC link into AC output power that is combined with the power from the generator stator to provide multi-phase power having a frequency maintained substantially at the frequency of the electrical grid bus (e.g. 50 HZ or 60 HZ).
The above system is generally referred to as a doubly-fed induction generator (DFIG) system, whose operating principles include that the rotor windings are connected to the grid via slip rings and the power converter controls rotor current and voltage. Control of rotor voltage and current enables the generator to remain synchronized with the grid frequency while the wind turbine speed varies (e.g., rotor frequency can differ from the grid frequency). Also, the primary source of reactive power from the DFIG system is from the RSC via the generator (generator stator-side reactive power) and the LSC (generator line-side reactive power). Use of the power converter, in particular the RSC, to control the rotor current/voltage makes it is possible to adjust the reactive power (and real power) fed to the grid from the RSC independently of the rotational speed of the generator. In addition, the generator is able to import or export reactive power, which allows the system to support the grid during extreme voltage fluctuations on the grid.
Typically, the amount of reactive power to be supplied by a wind farm to the grid during steady-state and transient states is established by a code requirement dictated by the grid operator, wherein a wind farm controller determines the reactive power demand made on each wind turbine within the wind farm. A local controller at each wind turbine receives and allocates the reactive power demand between the generator sources (e.g., between generator-side reactive power and line-side reactive power).
In general, the minimum speed (i.e. the cut-in speed) of the DFIG is decided based on the voltage limit imposed by the RSC (also referred to herein as the rotor voltage). Lowering or extending the minimum speed of the turbine increases the operating slip of the DFIG, which mandates the RSC to operate at a higher voltage. Therefore, the upper limit on the rotor voltage often determines the lowest possible rotor cut-in speed.
In addition to the minimum turbine speed, the reactive power requirement from the DFIG also effects the voltage at the terminals of the RSC. Further, as mentioned, most of the grid codes demand rated reactive power support during the entire operation of the wind turbine, i.e. from cut-in speed to rated speed. Providing the rated reactive power at the minimum turbine speed further forces the RSC to operate at a higher voltage.
One method of decreasing rotor voltage is to decrease the amount of over-excited (or capacitive) reactive power and, if possible, to increase the amount of under-excited (or inductive) reactive power produced by the generator. Further, most grid codes permit wind farms to produce less than full-load reactive power when the wind speeds are close to cut-in wind speed. For example, the Electric Reliability Council of Texas (ERCOT) generally allows for zero reactive power capability when the farm is operating below 10% of its rated power output. In contrast, the Federal Energy Regulatory Commission (FERC) requirements for the rest of the U.S. require reactive power capability only in proportion to the active power being produced by the wind farm.
Thus, an improved system and method for operating a wind turbine power system during low wind speeds to take advantage of the relaxation of the reactive power requirements at low wind speeds would be welcomed in the art. Accordingly, the present disclosure is directed to a system and method for operating a wind turbine power system during low wind speeds, but which also meet reactive requirements set by the grid codes that that level of power output.